1. Technical Field
This invention relates to plotters producing plots on rollable media and, more particularly, to a control system and method of operation for controlling the operation of a finished plot handling and rolling mechanism of a plotter. If the finished plot is not properly gripped for rolling, the rolling process is aborted and the plot ejected unrolled. During rolling, power to the driving mechanism is maintained at a level which will keep the media taut and prevent over powering. At the plot end, the media is made taut over the platen for consistent cutting. The trailing end is then accelerated at a level which will overcome static friction of the previously rolled portion and prevent buckling of the trailing portion.
2. Background Art
Pen plotters are popular so-called peripheral devices for use with computers, particularly in conjunction with applications programs such as CAD/CAM. They have an advantage over other forms of plotting of being simple in operation and construction and, therefore, lower in price. As shown in simplified form in FIG. 1, a basic pen plotter 10 after drawing the plot on the media 12 may simply move the media 12 over a support table 14 by means of a pinch roller 16 holding the media 12 against a driven drum 17 to eject the completed plot into a receiving bin 18. With large plot sheets, the completed plot can wrinkle and cause problems and poor print quality when later reproduction by a diazo process, or the like, is attempted. Attempting to roll the randomly-folded plot onto a old media roller for safe-keeping is a time-consuming and often frustrating process.
In more complex and costly devices such as electrostatic printers and plotters, a media rolling apparatus is sometimes included as part thereof. Such prior art attempts are large and expensive and, typically, incorporate a lot of surface area over which the moving media must be moved against the friction imposed thereby. Overcoming the frictional considerations adds to the complexity and cost of the apparatus.
A prior invention assigned to the assignee of this application is disclosed in U.S. Pat. No. 5,177,497 which issued 5 Jan. 1993. The apparatus of that patent and its manner of construction and operation is shown in FIGS. 2 through 4. The rolling mechanism 20 of that invention is intended to be mounted between the ejection point 22 and the receiving bin 18 in a pen plotter 10 such as that depicted in simplified form in FIG. 1. The exact placement and the specific shape of the components are, of course, a function of the pen plotter 10 into which the rolling mechanism 20 is incorporated.
Starting with the mechanism 20 in its open position as depicted in FIG. 2, there is a stationary portion 24 and a rotating portion 26 which is mounted on and rotates in combination with the shaft 28. The stationary portion 24 comprises a first input guide portion 30 connected to a first partially-cylindrical portion 32. Thus, as the media 12 moves along its exit path 36, it moves over the input guide portion 30 as depicted in FIG. 2. A single, horizontal, cylindrical drive roller 38 is disposed behind the input guide portion 30 in the center thereof and extends through a slot 40 provided in the supporting structure of the first input guide portion 30 for the purpose. Because of the low friction environment of this invention, one drive roller 38 in the center is sufficient and preferred, but several across the width of the media could be employed if desired. A media-presence sensor 42 is provided for sensing the presence of a leading edge of the media 12 at the location of the drive roller 38. The sensor 42 can be of any convenient type known to those skilled in the art for such purposes (mechanical or electrical) and simply provides a signal on its output line 44 to the control logic 46 when there is media 12 positioned below the drive rollers 38. Actually, the sensor 42 in the drawings is suggestive in nature only and its function may be accomplished through the use of components of the pen plotter itself. For example, it is suggested therein that the sensor 42 comprise a sensing of the position of the drum 17 of FIG. 1 by the logic 46 which knows the position of the media with respect to revolutions of the drum it has accomplished and, therefore, when it has employed the drum 17 to move the media 12 far enough towards the ejection point 22 to be below the drive rollers 38.
The rotating portion 26 comprises a second input guide portion 48 connected to a second partially-cylindrical portion 50. The second input guide portion 48 carries a free-wheeling roller 39 positioned to mate with the drive roller 38 with the media 12 therebetween as will be seen shortly. The two partially-cylindrical portions 32, 50 are pivotally joined at the location of the shaft 28. The shaft 28 is bi-directionally rotated by any appropriate drive mechanism 52 (motor or solenoid, with a motor being preferred) under the control of the logic 46 to rotate the rotating portion 26 between the open position of FIG. 2 and the closed position of FIG. 3.
In operation, with the mechanism 20 in its open position of FIG. 2, the drive rollers 38 are inactivated. As the leading edge of the media 12 passes over the drive roller 38, the fact is sensed by the sensor 42 as described above, which sends a signal to the logic 46 (or is known by the logic 46 through its interaction with the sensing process). Upon the leading edge of the media 12 having entered the rolling mechanism 20 to a point below the drive rollers 38, the logic 46 activates the drive mechanism 52 to rotate the rotating portion 26 to the closed position of FIG. 3.
As can be seen in FIG. 3, in the closed position the two input guide portions 30, 48 form a guide slot 54 extending into a cylindrical rolling chamber 56 formed by the two partially-cylindrical portions 32, 50. The drive rollers 38 (which preferably have a soft, resilient outer surface) are pressed lightly against the free-wheeling rollers 39 with the media 12 therebetween. The rotating portion 26 is maintained in the closed position of FIG. 3 by the drive mechanism 52. When the rotating portion 26 is in the closed position of FIG. 3, the logic 46 then activates the drive mechanism 60 which is connected to rotate the drive rollers 38. The drive rollers 38 then gently urge the media 12 forward through the guide slot 54 into the cylindrical rolling chamber 56 where it is smoothly rolled as shown in FIG. 3. When the logic 46 recognizes that the trailing edge of the media 12 has been reached, the logic 46 stops the drive mechanism 60 and activates the drive mechanism 52 in the opposite direction to open the rotating portion 26 to the open position of FIG. 4 whereupon the smoothly and neatly rolled media 12' falls into the receiving bin 18. The disclosure of the patent suggest that the position of the trailing edge of the media 12 does not have to be physically sensed as it is known to the logic 46 because the logic 46 knows how large the media 12 is in length and how much of the media 12 has been moved by the driving rollers 38.
While the apparatus of FIGS. 2-4 works well for its intended purpose, there are several aspects which can be improved for various reasons. For example, every component which can be eliminated reduces the cost of manufacture, which is a major driving force in a highly competitive business such as computer-driven plotters. Also, provision must be made for fail-safe operation--without adding to the cost of manufacture thereby. That is, provision must be made for the situation which may occur where, for some reason, the finished plotting media does not properly enter the rolling mechanism. This cannot simply be ignored.
Wherefore, it is an object of the present invention to provide a control system for a finished plot handler which provides accurate control of the apparatus with a minimum number of components.
It is another object of the present invention to provide a control system for a finished plot handler which provides fail safe operation in the event of a malfunction.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.